ALBERT

Description: The southern United States experienced extensional tectonics during the early Mesozoic development of the Gulf of Mexico and subsequent accumulation of up to 12 km of marine sediment. Few seismic stations have operated in the Gulf Coastal Plain for extended periods of time, and data for constraining ground-motion prediction models are sparse. Previous studies found that the region differs from other parts of eastern North America in terms of Lg attenuation. The Earthscope Transportable Array was in the south-central United States in 2010–2012, during a time of increased seismicity. That circumstance provided data for investigating Lg propagation. Sixteen earthquakes with magnitudes in the 3.2–5.6 range occurred in Oklahoma, Arkansas, and Texas and were recorded to distances of 1000 km. Stations in the coastal plain of Texas, Arkansas, Louisiana, and Mississippi exhibit strong attenuation of the Lg phase, compared with stations located to the north in the Ouachita orogenic belt and cratonic platform. Q associated with the crustal waveguide underlying the Gulf Coastal Plain was estimated as Q =365 f 0.62 . However, Lg blockage due to crustal thinning and anelastic attenuation due to the presence of several kilometers of sedimentary deposits occurs near the Gulf Coast. The average 0 value south of latitude 33° N is 96±10 ms. A remarkable linear correlation exists between the attenuation parameter 0 and the thickness of post-Jurassic sediments in the study region, such that stations near the coast of Texas, Louisiana, and Mississippi, which overlie ~12 km of sediments, exhibit values of 0 ~160 ms. This strong spatial dependence of attenuation has important implications and should be incorporated in the development of ground-motion prediction models using the approach suggested here. Online Material: Figures showing maps of regression residuals and receiver terms for 12 frequency bands.

Description: An eight-station seismic network was installed in August 2011 in the epicentral area of the 1886 Charleston, South Carolina, earthquake, within and near the town of Summerville. The network operated for one year and located 134 earthquakes with duration magnitudes ranging from –1.8 to 2.6. The earthquakes occurred in a tabular zone striking ~N186°E, with dip of about 43° to the west. The focal depths range from 13 km to the top of the early Mesozoic section at a depth ~1 km. The dimensions of the seismic zone are ~23 km along strike and 15 km down-dip. Many of the earthquakes are concentrated in the 2–6.5 km depth range, near shallow faults imaged on seismic-reflection profiles adjacent to the Ashley River. Our hypocenter relocations of 228 earthquakes occurring in the period 1977–2005 are consistent with the results obtained from the recent one-year station deployment. The 48 well-constrained focal mechanisms derived from the one-year deployment are predominantly reverse motion, and ~30% exhibit north–south-trending nodal planes consistent with the orientation of the seismic zone. The seismicity near Summerville has characteristics in common with the aftershock sequence of the 23 August 2011 Mineral, Virginia, earthquake. We interpret these results to indicate that the modern seismicity is the lingering aftershock sequence of the 1886 shock and that the mainshock occurred on a south-striking, west-dipping fault plane with predominantly reverse motion, possibly with a component of right-lateral strike-slip. Most moment release probably occurred at depths greater than 6 km. Online Material: Tables listing hypocenters and focal mechanisms, and an animation showing hypocenter profiles oriented along different azimuths.

Description: 〈span〉〈div〉Abstract〈/div〉The Atlantic and Gulf Coastal Plain in the southern and southeastern United States contains extensive Cretaceous and Cenozoic sedimentary sequences of variable thickness. We investigated the difference in response of sites in the Coastal Plain relative to sites outside that region using Fourier spectral ratios from 17 regional earthquakes occurring in 2010–2018 recorded by the Earthscope transportable array and other stations. We used mean coda and 〈span〉Lg〈/span〉 spectra for sites outside the Coastal Plain as a reference. We found that Coastal Plain sites experience amplification of low‐frequency ground motions and attenuation at high‐frequencies relative to average site conditions outside the Coastal Plain. The spectral ratios at high frequencies gave estimates of the difference between kappa at Coastal Plain sites and the reference condition. Differential kappa values determined from the coda are correlated with the thickness of the sediment section and agree with previous estimates determined from 〈span〉Lg〈/span〉 waves. Averaged estimates of kappa reach ∼120  ms at Gulf coast stations overlying ∼12  km of sediments. Relations between 〈span〉Lg〈/span〉 spectral ratio amplitudes versus sediment thickness in successive frequency bins exhibit consistent patterns, which were modeled using piecewise linear functions at frequencies ranging from 0.1 to 2.8 Hz. For sediment thickness greater than ∼0.5  km, the spectral amplitude ratio at frequencies higher than approximately ∼3  Hz is controlled by the value of kappa. The peak frequency and maximum relative amplification at frequencies less than ∼1.0  Hz depend on sediment thickness. At 0.1 Hz, the mean Fourier amplitude ratio (Coastal Plain/reference) is about 2.7 for sediment of 12 km thickness. Analysis of residuals between observed and predicted ground motions suggests that incorporating the amplification and attenuation as functions of sediment thickness may improve ground‐motion prediction models for the Coastal Plain region.〈/span〉

Description: We used aftershocks of the 2011 Mineral, Virginia, earthquake to study geometrical spreading at hypocentral distances less than 60 km in the central Virginia seismic zone. Sixty-nine aftershocks, occurring from 25 August 2011 through 24 December 2011, provided the data. We used the coda-normalization method to estimate the attenuation coefficient associated with geometrical spreading. We filtered the time-domain signals in several octave-wide frequency bands and examined attenuation of peak S -wave amplitude in the 1.0–30.0 Hz frequency range. Amplitude was assumed to decrease as a function of hypocenter distance R according to R – . The coefficient of attenuation was examined for the three-component S -wave amplitudes, with corrections for SH and SV radiation patterns. We observed no systematic frequency dependence of . The coefficient of attenuation for the radial and transverse components, assuming infinite quality factor Q , derived as a weighted mean over the entire range of frequencies (1–30 Hz), are both 1.51±0.05. The weighted mean value of the attenuation coefficient on the vertical component over the same range of frequencies is 1.45±0.05, slightly less than for the horizontal components. We corrected the data assuming three Q models. The estimated geometrical spreading coefficients are in the 1.30–1.46 range, depending on the assumed Q model and component, which is only slightly less than the estimates of determined assuming infinite Q . The estimated attenuation coefficients differ significantly from the value of 1.0 expected for a whole space. The results for the horizontal components are in agreement with previous full-wavefield modeling. However, the observed vertical-component attenuation is substantially less than that predicted by the synthetics. The depths of the earthquakes are less than 8 km, so these results may not be representative of geometrical spreading in parts of eastern North America where earthquakes occur at greater depths. Online Material: Table of earthquake hypocenters and focal mechanisms.

Description: Horizontally layered velocity models were used with point-source and finite-fault sources to investigate geometrical spreading and the relative amplitudes of vertical and horizontal ground acceleration within 120 km of the source. Full-wave-field simulations were done for a range of focal depths and for strike-slip and reverse focal mechanisms. The attenuation of the geometric mean of randomly oriented horizontal-component maximum acceleration amplitudes, averaged over all azimuths, significantly exceeds the theoretical geometrical spreading for far-field body waves in a homogeneous whole space for hypocentral distances less than approximately 60 km. The behavior of the vertical component is different from the horizontal: vertical attenuation near the epicenter is greater and is more dependent on source mechanism and depth. Because of the rapid near-source decay of the direct S wave, reflections from the mid-lower crust and Moho control the maximum amplitude of the vertical-component acceleration in the 60–120-km hypocenter distance range, resulting in a flattening of the vertical amplitude-distance relation. Near-source vertical maximum amplitudes averaged over all source–receiver azimuths tend to be less than the geometric mean horizontal amplitude for strike-slip focal mechanisms, but, near the source for reverse faults, the azimuthally averaged vertical-component amplitude exceeds that of the geometric mean horizontal. The modeling indicates that similar vertical- and horizontal-component geometrical spreading and approximately constant horizontal/vertical amplitude ratios observed in connection with the Lg phase at distances greater than approximately 100 km in eastern North America may not hold at smaller distances. Ground-motion prediction models for the vertical component near the source may need to incorporate strong geometrical spreading and dependence on radiation pattern.

Description: The aftershocks of the 23 August 2011 M w  5.7 Mineral, Virginia, earthquake were recorded by 36 temporary stations installed by several institutions. We located 3960 aftershocks from 25 August 2011 through 31 December 2011. A subset of 1666 aftershocks resolves details of the hypocenter distribution. We determined 393 focal mechanism solutions. Aftershocks near the mainshock define a previously recognized tabular cluster with orientation similar to a mainshock nodal plane; other aftershocks occurred 10–20 km to the northeast. A large percentage of the aftershocks occurred in regions of positive Coulomb static stress change, and ~80% of the focal mechanism nodal planes were brought closer to failure. However, the aftershock distribution near the mainshock appears to have been influenced strongly by rupture directivity. Aftershocks at depths less than 4 km exhibit reverse mechanisms with north-northwest-trending nodal planes. Most focal mechanisms at depths greater than 6 km are similar to the mainshock, with north-northeast-trending nodal planes. A concentration of aftershocks in the 4–6 km depth range near the mainshock are mostly of reverse type but display a 90° range of nodal-plane trend. Those events appear to outline the periphery of mainshock rupture, where positive Coulomb stress transfer is largest. The focal mechanisms of aftershocks at depths less than 4 km and those greater than 6 km, along with the mainshock, point to the possibility of a depth-dependent stress field prior to the occurrence of the mainshock. Analysis of earthquake occurrence using a new magnitude scale ( ) indicates a Gutenberg–Richer law b -value of 0.864 and an Omori law p -value of 1.085, indicative of a typical aftershock sequence. Online Material: Catalogs of aftershock location, magnitude, and focal mechanisms.

Description: INTRODUCTION The seismic design of structures sometimes necessitates the use of synthetic strong ground motion time histories (European Committee for Standardization 2003; International Code Council 2000). Toward this end, the stochastic method with a point-source representation of the seismic source is a fast and efficient way to generate synthetic time histories (e.g., Boore 2003). While other modeling procedures may be more refined and physically realistic (e.g., full finite-fault simulations), they generally require a larger number of input parameters for which calibration relations do not exist (see Douglas and Aochi 2008, and references therein). Even for well-recorded earthquakes, these parameters are difficult to develop calibration relations for because of (1) the observed variability in the results from earthquake source process inversion and its dependence on available data (e.g., Custódio et al. 2005) and (2) the overall lack of confidence in even the best estimates of the parameters controlling fault rupture (Monelli and Mai 2008; e.g., Monelli et al. 2009). The simple seismological models also have advantages over empirical ground motion predictive equations (GMPE) (Abrahamson and Shedlock 1997; Power et al. 2008), although the forms of the source, path, and site functions in the simple seismological models may be somewhat less flexible than empirical GMPEs when trying to fit existing data. Seismological models can provide information on the physical nature of the parameters controlling the strong motion while the GMPEs cannot (e.g., Boore 2003; Olafsson et al. 2001; R. Sigbjörnsson and Ambraseys 2003). Moreover, despite the increase in strong motion recordings over the last couple of decades, and presumably the corresponding increase in knowledge, there are still considerable differences among the median ground motions estimated using the various GMPEs. These differences are a measure of epistemic uncertainty and have not been...

Description: An earthquake-induced sand blow was discovered in a shallow trench dug on the premises of the Colonial Dorchester State Historical Site in Summerville, South Carolina. A comparison of its location with available seismicity, seismic reflection, and shallow geological and geomorphologic data suggests that the sand blow was associated with a splay of the currently active Sawmill Branch fault zone. This is the first sand blow to be directly associated with a specific fault in the Middleton Place-Summerville seismic zone, the source zone for the Charleston earthquakes. Geotechnical and vibracore data revealed that the source sand is [~]3 m thick and the top of the sand is at a depth of [~]2.5 m below the ground surface. The sand blow was associated with a pre-1886 earthquake that occurred possibly 3,500 YBP or earlier, with an estimated maximum magnitude of 5.6.